Non-aqueous lithium batteries have long been known to offer certain advantages over the more conventional aqueous systems. These advantages generally include higher operating voltages per cell, superior shelf-life and charge retention, and higher gravimetric and volumetric energy densities. Primary lithium batteries have been available commercially for many years in various sizes for many consumer electronics applications. Secondary or rechargeable batteries are also commercially available, but until recently these have been limited to small sizes (e.g. coin cell size). Larger rechargeable lithium batteries have historically proven not to be safe enough for consumer applications.
A larger type of rechargeable battery, known as a lithium-ion or rocking chair battery, has recently become a state-of-the-art power source for consumer electronics devices. Two companies, Sony Energy Tec and A&T Battery, presently manufacture lithium-ion batteries employing a lithium cobalt oxide compound as the cathode and a carbonaceous material as the anode. These batteries have significantly greater energy density than either conventional Ni-Cd or Ni-Metal Hydride (Ni-MH) batteries. Furthermore, since the lithium-ion batteries have an average discharge voltage of about 3.6 volts, a single Li-ion battery can be used to replace three series connected Ni-Cd or Ni-MH batteries.
Preferred materials for use as cathodes in both primary and secondary lithium batteries include members of the class consisting of transition metal oxides or lithiated transition metal oxides. Vanadium oxide and manganese oxide cathode materials are particularly common. Lithiated transition metal oxides are at present the preferred cathode material for use in Li-ion batteries. Unlike other Li batteries where Li is usually incorporated into the anode on assembly (often directly as Li metal or in the form of a Li alloy), in a conventional Li-ion battery the lithium transition metal oxide cathode is the only source of lithium available for battery operation. Thus, for optimum battery capacity, it is desirable to use a lithiated transition metal oxide cathode containing substantial amounts of lithium that can be extracted and re-inserted reversibly. Additionally, it is desirable that the lithiated cathode material be completely stable in air for manufacturing simplicity. Examples of suitable cathode materials include both LiCoO.sub.2 and LiNiO.sub.2 (described in U.S. Pat. No. 4,302,518) and LiMn.sub.2 O.sub.4, (described in U.S. Pat. No. 4,246,253). Currently, only Li-ion batteries employing Co based cathodes are available. Since Co is relatively rare and is hence expensive, competitive, less expensive alternatives are desirable. Ni based cathodes can be less expensive but both Co and Ni compounds are considered potential cancer causing agents. Being relatively inexpensive and less of a health concern, Mn based compounds appear to be attractive potential alternative cathode materials.
For these reasons, lithium manganese oxides have been extensively studied for use as cathode materials for rechargeable lithium batteries. These oxides typically can have stoichiometries wherein the Li:Mn ratio ranges from 0 to 2, and the O:Mn ratio ranges from 2 to 3. In a rechargeable battery, the capacity is a function of how much lithium can be reversibly inserted into the host oxide cathode. For some lithium-manganese oxides, almost all the available lithium can be reversibly inserted.
The spinel materials Li.sub.4 Mn.sub.2 O.sub.4 (described in M. M. Thackeray et al, J. Electrochem. Soc. 137, 769 (1990)) and the aforementioned LiMn.sub.2 O.sub.4 both contain at least 1/2 mole of Li per mole of manganese and are hence attractive materials for use in Li-ion batteries. If all the lithium in these compounds could be removed and re-inserted reversibly, these materials would have reversible capacities of 148 and 216 mAh/g respectively. Recently, as in T. Ohzuku et al, Chemistry Express 7, 193 (1992), a low temperature form of orthorhombic LiMnO.sub.2, called LT-LiMnO.sub.2, has also been found to be an attractive electrode material. Again, if all the lithium in LT-LiMnO.sub.2 could be removed and re-inserted reversibly, it would have a reversible capacity of 285 mAh/g.
However, not all the lithium in these Li-Mn-O compounds can always be removed electrochemically. In fact, A. Momchilov et al, J. Power Sources 41, 305 (1993) show that the reversible capacity of LiMn.sub.2 O.sub.4 depends critically on synthesis conditions, with the best materials being made between 650.degree. C. and 750.degree. C. Momchilov et al. also show that the surface area of the material synthesized decreases with increasing synthesis temperature. Higher surface area materials are attractive however for high discharge rate capability in batteries. Thus, it would appear that both reversible capacity and surface area cannot be optimized independently during this synthesis. Low temperature synthesis can produce the highest surface areas, but high temperature synthesis results in the highest reversible capacity.
The effect of higher synthesis temperature on reversible capacity was also noted in U.S. Pat. No. 5,211,933 where the capacity for the invention LiMn.sub.2 O.sub.4 made @300.degree. C. increases from about 75 mAh/g to about 120 mAh/g for conventional material made @800.degree. C. However, the desirable advantages of the method of the invention are achieved at temperatures below 600.degree. C.
The desirable material LT-LiMnO.sub.2 is made at temperatures below 350.degree. C. Higher temperature treatment even in an inert atmosphere results in a conversion of this material to crystalline LiMnO.sub.2 with poor electrochemical behaviour. The reversible capacity of the LT-LiMnO.sub.2 material reported in the aforementioned paper by T. Ohzuku et al. was about 190 mAh/g.
As is clear from the preceding, certain advantages can be realized by synthesizing Li-Mn-O compounds at low temperature. However, the resulting material can have less than optimal reversible capacity. Ideally, obtaining the certain advantages in combination with optimum reversible capacity is preferred.
Standard methods exist for the controlled removal of oxygen from solid oxide compounds. A preferred method in the art is to heat such a compound in a reducing gas mixture wherein a gas such as H.sub.2, NH.sub.3 or the like is used to react with oxygen in the compound thereby forming gaseous reaction products which can be easily removed. Such a method is described in U.S. Pat. No. 5,240,794 to prepare desirable Li-Mn-O cathode precursors for use in lithium batteries. Said method can be used to augment the total amount of lithium loaded into a Li-ion battery. However a change in phase or phases of the Li-Mn-O compounds is often involved both in the method reduction step and in subsequent use in a battery wherein the invention precursor is delithiated irreversibly to act as a cathode after a first activating recharge.